This invention relates to a photographic light-sensitive material having a silver halide emulsion layer stabilized against over-development fogging.
When a silver halide photographic light-sensitive material is subjected to development treatment under strong conditions, for example, at relatively high temperatures or with a highly active developer (having a high temperature and a high pH) for a very short time, there is the possibility of silver halide grains containing no latent image centers being also reduced.
The fog produced by the undesirable reduction of unexposed silver halide grains under the above conditions ordinarily appears especially strongly at the time of completion of development and is called over-development fog.
Antifoggants known to be effective in restraining the over-development fog are mercury compounds and heterocyclic mercapto compounds. These antifoggants reduce fog during development or over-development, but they also have the drawback of decreasing the sensitivity of silver halide photographic light-sensitive materials when added in an amount enough to reduce the fog.
The fundamental disadvantage of direct addition of heterocyclic mercapto compounds which are per se especially effective on over-development fog to silver halide emulsions is that they are already completely active at the time of addition thereof, namely, they are already completely active during preparation, storage and development.
They, they exert undesirable desensitization action during the periods of preparation and storage of the photographic light-sensitive materials.
In order to obviate these problems an attempt has been made to protect the mercapto group of these compounds by a suitable hydrolyzable group thereby to cause them to remain inactive for the period during which the action thereof is not desired to exert (the whole period prior to development including preparation period) while the activity is restored by hydrolysis with alkali during development. Such substituent groups are normally thioesters or thioethers of the mercapto antifoggants.
As the substituent groups of thioester type, thioesters of carboxylic acids, sulfonic acids and carbonate derivatives are disclosed in many patents, e.g., German Pat. No. 1,597,503, U.S. Pat. No. 3,260,597 and German Laid-Open Patent Application No. 2,061,972. However, these thioesters have the defect that they are hydrolyzed in an alkali medium of developing solution, but they also undergo gradual partial hydrolysis in neutral or weakly acidic pH region. Therefore, those thioester type antifoggants can be added in an inactive form to emulsion, but they may exert undesired desensitizing action due to the partial hydrolysis if the periods of preparation of emulsion and storage of photographic light-sensitive material are considerably long.
On the other hand, antifoggants having thioether type substituent, for example, those disclosed in U.S. Pat. No. 2,981,624 and U.S. Pat. No. 3,260,597 and German Pat. No. 1,173,796 are stable in neutral or weakly acidic medium, but they do not reproduce the original mercapto antifoggants at all or reproduce only very slowly and thus cannot effectively prevent the over-development fog.
Furthermore, those which are disclosed in U.S. Pat. No. 3,674,478, namely, compounds which release a mercapto antifoggant and form quinone-methide or naphthoquinone-methide in the presence of alkali rapidly release the mercapto antifoggant in the presence of alkali and are effective in prevention of over-development fog, but are somewhat unstable in a weakly acidic medium and only slowly release the antifoggant.
Therefore, when the period of from preparation to coating of emulsion (pH of the emulsion is weakly acidic) is sufficiently long, the antifoggant is released resulting in the undesired desensitization.
Furthermore, even if said compound in a complete form to emulsion, a greater loss of sensitivity is brought about as compared with the sensitivity obtained without the antifoggant although some increase may be seen as compared with the sensitivity when an active mercapto antifoggant is added.
Thus, addition of such compounds makes only a slight contribution to increase in sensitivity of silver halide photographic light-sensitive materials.